Polymer electrolyte fuel cells have a proton conductive membrane placed between a fuel electrode and an air electrode. Since they can be reduced in size and weight, they are expected to be used as on-board automotive fuel cells.
As a proton conductive membrane for a polymer electrolyte fuel cell, Nafion (a trade name by DuPont) has been used. However, Nafion can hardly act to operate a cell at higher temperatures since it is poor in thermal stability and chemical stability. In order to produce satisfactory output in a Nafion-incorporating fuel cell even at lower temperatures, it is needed to use a large amount (about 40 to 60% by weight based on the weight of an air electrode) of a platinum-supported catalyst in the air electrode, causing the increase in cost of the fuel cell. Further, Nafion in itself is also expensive. Development of new proton conductor materials as alternatives to Nafion has currently been pursued, but has not yet reached the practical stage (Non-patent reference No. 1, Patent reference Nos. 1 and 2).
On the other hand, solid acid catalysts had been actively studied, since they do not require any process for their separation and collection such as neutralization and removal of salts and can produce desired products in an energy-saving manner without production of unwanted side products (non-patent reference No. 2). Consequently, solid acid catalysts including zeolite, silica-alumina and water-containing niobium have had a great success in the field of chemical industry and offered great benefits to society. Nafion mentioned above is also a very strong solid acid having hydrophilic properties (i.e., an ultrastrong solid acid) and is already known to act as an ultrastrong acid having an acid strength higher than those of liquid acids. However, polymeric solid acid catalysts including Nafion have such disadvantages that they are sensitive to heat and that they are too expensive to use on an industrial scale. Therefore, it is difficult to design industrial processes for the production of solid acid catalysts which are advantageous compared to liquid acid catalysts from the viewpoint of performance and cost, and current chemical industry seems to predominantly depend on liquid acid catalysts. Under these circumstances, the advent of solid acid catalysts which outperform liquid acids in performance and cost has been demanded.
[Non-patent reference No. 1] HIGH TEMPERATURE MEMBRANES FOR SOLID POLYMER FUEL CELLS, ETSU F/02/00189/REP, Contractor Johnson Matthey Technology Centre, Prepared by Martin Hogarth Xavier Glipa, Crown Copyright, 2001, Pi-15, particularly page 4, TableI 1.
[Non-patent reference No. 2] Ishihara, K; Hasegawa, A; Yamamoto, H. Angew. Chem. Int. Ed. 2001, 40, 4077.
[Patent reference No. 1] Japanese Patent Application Laid-open No. 2003-217341.
[Patent reference No. 2] Japanese Patent Application Laid-open No. 2003-342241]